C128 Video Output and RGB to B/W Composite Diagram
I just hooked my C128 to my 15" television set. The same cable can be used
for both 40 and 80 column screen, as I have a switch in it.
This cable is based on an article in C=Lehti 2/89. It had some inaccuracies,
and it is in Finnish. So I'll describe the cable here.
The RGBI connector looks like following, when looking to the machine's rear
side from outside:
5 4 3 2 1
9 8 7 6
(The User's guide and the C=Lehti article used the mirror image of this,
which confused at least me.)
The pins are as follows:
1 GND ground
2 GND ground
3 R red
4 G green
5 B blue
6 I intensity
7 VIDEO composite video
8 HSYNC horizontal sync
9 VSYNC vertical sync
My television has a 21-pin Scart connector, which is used in Europe. It is
a special type of connector that has rectangular plates as pins. The pins
are surrounded by a pentagonal metal frame. It looks like the following:
_________________________________________
| |
| 1 3 5 7 9 11 13 15 17 19 |
| |
| \
| 2 4 6 8 10 12 14 16 18 20 \
--------------------------------------------
The needed pins are:
4, 5, 9, 13, 17 ground
2 audio, right channel
6 audio, left channel
7 blue
11 green
15 red
20 video
16 fast blank
The Scart connector does not allow use of HSYNC and VSYNC signals, so it
uses a video signal to synchronize the RGB picture. As the video signal
can be used also without RGB, the "fast blank" signal is needed to enable
RGB signal, unless you have a switch in your television to enable RGB.
When the television or monitor gets a positive voltage to the "fast blank" pin,
RGB will be enabled. The C=lehti article instructed to tie VSYNC to this
pin through a 220 ohm resistor to provide the voltage, but it was too weak for
my TV. So I temporarily used a 9 V battery to get some color on the screen.
Finally I hooked that pin to the cassette port's +5V output.
The television expects analog RGB, but the C128 outputs digital RGB. The
signals can be converted to analog using six resistors:
digital analog
R ---- R1 ---- R ---- R4 ---+
|
G ---- R2 ---- G ---- R5 ---+
|
B ---- R3 ---- B ---- R6 ---+
|
I --------------------------+
The article suggested R1, R2 and R3 to be 470 ohms and R4, R5 and R6 to be
680 ohms. You can experiment with other values to get good-looking colors
on the screen.
On some C128's, the RGBI connector's VIDEO signal might be actually 40 column
screen's video signal, or the C=Lehti article is simply wrong when claiming
that you can get the 40 column screen via the RGBI connector. In any case,
you get sound and 40 column screen from the 8-pin VIDEO connector. You can
plug a 180 degree 5-pin DIN connector to it. The pin 2 is ground, 4 is video
signal and 3 is audio output.
Finally you have to add a 2*ON-ON switch to switch between 8563 and 8566
screen. Connect it as follows:
/
RGBI Video (7)-----o/ 80 column screen
/|
Scart Video (20)----/ |
|
VIC Video (4)-----o| 40 column screen
|/
voltage supply-----o/
/
Fast blank (16)-R7-/ R7=220 ohms
o
As mentioned above, you might be able to use VSYNC as voltage supply. If
the cassette port's +5V pin is not enough for your TV or monitor, use a
9 V battery or take a +9V or +12V lead from your computer.
Be careful with the 80 column mode. If you reset the computer to 64 mode,
the VDC screen will be out of syncronization, and your monitor may start
to smoke if you leave the cable in 80 column mode for several seconds.
Part list:
Quantity Quality
======== =======
3 470 ohm resistors
3 680 ohm resistors
1 220 ohm resistor
1 Scart connector
1 D9S connector
1 5-pin 180-degree DIN plug
1 2*ON-ON switch
To connect your C128's 80 column screen to a CGA monitor, simply connect all
wires. I have planned to connect it to my multisync SuperVGA monitor. I think
that this goes by converting the digital RGB to analog and by providing the
monitor with VSYNC and HSYNC signals. So far I haven't done this, as I don't
have the 15 pin connector, and I do not know its pinout.
Have fun connecting!
--
Marko Mäkelä | M H
University of Helsinki | a e
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